Image Processing Automates Eyeglass Fitting for Rodenstock

16. Aug 2013
References

Imagine that you wear varifocal glasses and need new ones. You go to an optometrist and, without time-consuming adaptations, measuring or trying on frame after frame, you look at yourself in a mirror and - voila! - your chosen frame is on your nose.

Fig. 1: In a calibrated system, the geometric relationship of the coordinate systems of the center camera and the side camera are determined by triangulation. The

Fig. 2. The pupils can be found by the image processing software, which uses the three reflections of the illumination around the pupil. After the pupils are found, the position crosses are set by the software.

Fig. 3: The coordinates from the camera that takes the front image are translated and projected onto the image taken by the other camera. The optometrist can then finalize the position of the pupil center cross mark and the cross marks at the glasses.

International eyeglass manufacturer Rodenstock GmbH of Munich has developed an optometrist service terminal that can do just that. It enables automatic calculation of eyeglass frames and lenses customized to fit an individual's face - all in about a second.

The terminal, called the ImpressionIST, performs several tasks. It provides interactive information about the eyeglasses, presents frames from which the customer can choose, calculates the ideal dimensions of the glasses via a three-dimensional video system, and offers consultation regarding the quality of the glasses. The technical requirements of the system are targeted to the aesthetic aspects of choosing glasses.

Exacting images

The patent-pending 3D video system, which measures the customer's face, must make the fitting procedure as easy as possible. Thus, the 3D measurements are done with the customer looking into a semitrans-parent mirror that is 75 cm away. The imaging system, mounted behind the mirror, is hidden from view. It uses two cameras from Allied Vision Technologies to take images simultaneously. After the measurements are concluded, the optometrist can go over the images with or without the customer.

One camera captures the face from the front and the other, from below and the side (Figure 1). These create aesthetically pleasing images, while being suitable for analysis.

The images are processed based on the software library HALCON from MVTec Software GmbH of Munich. The software delivers the real-world 3D coordinates necessary for manufacturing the frames and lenses. By acquiring a calibration plate before the first start-up of the ImpressionIST, inner and outer camera parameters can be appointed to deliver the coordinates.

The analysis begins with a position cross mark placed at each pupil's center on the image of the front of the face (Figure 2). The software uses information about the camera position, illumination and reflection from the cornea to calculate the centers.

To make measuring as comfortable as possible and to ensure the customer's elemental posture during image acquisition, no flash is used. For this reason, three 11-W lamps from Osram are integrated into the system, arranged to find the pupils' centers from the reflections on the corneas. The showroom should be illuminated with diffuse light.

The final position can be adjusted by the optometrist. The brims of the glasses are computed by edge extracting each glass's front. Other cross marks are positioned at the point where the extensions of the straight lines through the pupils' cross marks meet the brims. These marks also can be adjusted. The same positions are determined for the image from the other camera, and the already adjusted cross marks from the front image are projected onto it (Figure 3). Then, the cross marks can be automatically transferred into each other.

For Rodenstock, it was important that the optometrist maintain the ability to adjust the positioning, although the image processing software could do it. After the optometrist approves the positioning, the 3D coordinates of the corresponding cross marks are computed, taking the calibration into consideration.

The computation delivers all the parameters needed to manufacture the frames, including monocular pupil and corneal vertex distance, pantoscopic tilt, face-form angle, fitting point height, decentration (of the fitting point), boxed data (lens width and height), distance between lenses, frame shape and optimized lens diameter. The results are documented in a hard copy that includes illustrating figures and that is optimized to allow grinding of the lenses.

Rodenstock's tests of the system by Rodenstock have found that the measurements are accurate and repeatable, depending on the customer's ability to repeat a position. The reproducibility using the system was 1.5 times higher than when using a free distal view.

The system is currently available only for Rodenstock consultant optometrists.